1. Field of the Invention
The invention relates to a polymer composition comprising pH-sensitive fluorescent dyes, to an ionic strength-independent optical sensor for pH value determination that contains the composition in the form of a membrane on a transparent support material, and also to an optical process, according to the fluorescence method, that renders possible highly accurate pH value determination independently of the ionic strength of the test solution. The process is especially suitable for the determination of the pH value of physiological solutions, especially for the determination of the pH value of blood.
2. Description of Related Art
It is known that the pK.sub.a value of an indicator varies with the ionic strength of a solution and that that variation depends on the level of the charge at the indicator. For example, it has already been proposed in DE-A-3 430 935 to determine computationally the ionic strength and the pH value from the difference between the measured values of two sensors having different ionic strength dependence of which one exhibits as low as possible an ionic strength dependence, after calibration of said sensors with known test solutions. The sensor described therein that is almost independent of the ionic strength does not lie exactly within the physiological pH range and has a low resolution. The construction of those sensors is effected without embedding into a polymer matrix and consequently has the disadvantage that the dye is in direct contact with the test solution. The fluorescent dye of the sensors, which is the same in each case, is in that arrangement immobilised directly on the surface of glass supports by way of bridging groups, one of the sensors containing additional charges for achieving a high polarity and ionic strength dependence and the other sensor being so modified that it is essentially non-polar, hydrophobic and independent of the ionic strength. A quite considerable disadvantage of those sensors is that the fluorescent dye is exposed directly to external influences of the test solutions, and both physical influences (for example dissolution of the dye, deposits on the surface) and chemical influences (decomposition of the dye) quickly make the sensors unusable. In addition, in the case of excitations in an evanescent field, interference between the evanescent measuring field and the fluorescence of the test sample cannot be completely avoided, which reduces the accuracy of the measurement. The response time of those sensors is on the other hand short, since the fluorescent dye bonded to the surface immediately comes into contact with the test solution. The sensitivity is regarded as adequate.
The method of optical pH determination using two sensors that respond to different extents to the ionic strength of a test solution is expensive in respect of apparatus and a subsequent, additional calculation step has to be carried out.